The connective tissue changes that accompany intestinal Crohn's disease have received little attention from pathologists. This is particularly so with fat hypertrophy, and yet surgeons have long recognized the phenomenon of fat-wrapping in the intestines and used it to delineate the extent of active disease. A consecutive, unselected series of 27 intestinal resections performed on 25 patients for histologically confirmed Crohn's disease was studied to correlate fat-wrapping with other clinicopathological features. Fat-wrapping was identified in 12 of 16 ieal resections and in seven of 11 large bowel resections. It correlated closely with transmural inflammation and there was a relationship between fat-wrapping and other connective tissue changes including fibrosis, muscularization and stricture formation. Morphometry demonstrated that there was true hypertrophy and that fat-wrapping does not relate solely to bowel wall shrinkage. There was correlation with ulceration but in 11 cases macroscopic ulceration extended beyond the fat-wrapping and in six to surgical resection margins. The pathological features of 225 small intestinal resections were reviewed and fat-wrapping was seen only in Crohn's disease. Fat-wrapping correlates best with transmural inflammation and represents part of the connective tissue changes that accompany intestinal Crohn's disease. Findings also suggest that fat-wrapping alone should not be used as an accurate marker of disease extent at the time of surgery.